MARINE MAMMAL SCIENCE, 32(3): 826–838 (July 2016)
© 2016 Society for Marine Mammalogy
DOI: 10.1111/mms.12302
Seasonality of Antarctic minke whale (Balaenoptera
bonaerensis) calls off the western Antarctic Peninsula
TALIA DOMINELLO1 AND ANA SIROVIC , Scripps Institution of Oceanography, University of
California San Diego, 9500 Gilman Drive, La Jolla, California 92093-0205, U.S.A.
Abstract
The Antarctic minke whale (Balaenoptera bonaerensis) is a difficult species to study
because of its low visual detectability and preference for living within the sea ice
habitat, accessible only by ice-strengthened vessels. Recent identification of the
Antarctic minke whale as the source of the seasonally ubiquitous bio-duck call has
allowed the use of this sound, as well as downsweeps, to investigate seasonality
trends and diel patterns in Antarctic minke whale call production, and their relationship to sea ice cover. Passive acoustic data were collected using an autonomous
Acoustic Recording Package (ARP) off the western Antarctic Peninsula. Bio-duck
calls were classified into four distinct call variants, with one variant having two subtypes. Bio-duck calls were detected between April and November, with increasing
call duration during the austral winter, indicating a strong seasonality in call production. Downsweeps, which were also attributed to Antarctic minke whales, were
present throughout most months during the recording period, with a peak in July,
and an absence in March and April. Both bio-duck and downsweeps were significantly correlated with sea ice cover. No diel patterns were observed in bio-duck calls
or in downsweep call production at this site.
Key words: passive acoustics, minke whale, Balaenoptera bonaerensis, western Antarctic Peninsula, bio-duck, vocalizations, seasonality.
Passive acoustic monitoring (PAM) is an indispensable tool to study marine
mammal distributions (Van Parijs et al. 2009). One tribulation, however, is that it
lacks the visual identification component for linking unknown sounds to a specific
species. However, even without confirmed knowledge of the sound source, much
can be learned about the distribution and seasonality of a sound occurrence (e.g.,
Thompson and Friedl 1982), and once sounds have been attributed to a particular
species (Rankin and Barlow 2005), this information can greatly augment our
understanding of the biology of a species. An example of one regionally common
sound with unknown origin was the bio-duck call, first recorded in 1964 in the
Ross Sea in the presence of leopard seals (Hydrurga leptonyx) preying upon
penguins.
The bio-duck occurred at a rhythmic interval with regularly repeated groups of
three pulses separated by a time interval equivalent to two pulses. These features,
highly stereotyped sequence of frequency modulated short duration pulses and bouts
lasting many hours, were shared with calls recorded from minke whales elsewhere
1
Corresponding author (e-mail: [email protected]).
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DOMINELLO AND SIROVIC
827
(Mellinger et al. 2000), thus the bio-duck call is thought to be produced by minke
whales (Mellinger et al. 2000, Van Opzeeland 2010). Antarctic minke whales (Balaenoptera bonaerensis) are known to be regularly sighted in Antarctic waters, so the
Antarctic minke whale was thought to be a good candidate for production of the
bio-duck call (Van Opzeeland 2010). However at the time, no specific evidence
existed to rule out fish or other species as the source of the call (Van Opzeeland
2010). The conclusive link was obtained after a tag deployed on an Antarctic minke
whale in Wilhelmina Bay off the western Antarctic Peninsula, recorded the bio-duck
(Risch et al. 2014a). The calls recorded on a tag consisted of a series of 5–12 pulses,
produced in regular sequences at an interval between sequences of 3.1 s. The individual downswept pulses making up the bio-duck had a mean duration of 0.1 s and
mean peak frequency 154 13 Hz (Risch et al. 2014a). The presence of the bio-duck
in long-term recordings can shift between months, although it always occurs during
the austral spring and winter, and monthly call rates differ between years in the
western Antarctic (Van Opzeeland 2010). The bio-duck call has also occurred during
the same time period off Western Australia (Matthews et al. 2004).
Antarctic minke whales also produce low frequency downsweeps (Schevill and
Watkins 1972, Risch et al. 2014a). These sounds consist of a single downsweep from
130 Hz to 60 Hz. In one study, downsweeps were produced just before or after surfacing while animals were within pack ice (Schevill and Watkins 1972). Despite
downsweeps having been known and recorded from minke whales for a number of
years, little is known about their functionality (Edds-Walton 2000, Gedamke et al.
2001). Downsweep calls from other baleen whale species are suggested to serve a
social function or to maintain spacing and contact with separated individuals (EddsWalton 1997, 2000).
There are two genetically distinct populations of Antarctic minke whales. The two
populations overlap with no sharp boundary between them, but one predominates in
the east and the other in the west (IWC 2013). The use of bio-acoustics in remote
areas has the potential to help identify a more distinct line (Van Parijs et al. 2009). A
particularly high abundance of minke whales has been identified in the northwestern
Weddell Sea, in the pack ice bordering the Scotia-Weddell Confluence (Ainley et al.
2012). Additionally, Antarctic minke whales off the western Antarctica Peninsula
(WAP) are found in high densities year-round very close to shore, with the population estimated at 1,544 individuals (95% CI: 1,221–1,953) (Thiele et al. 2004,
Williams et al. 2006). Recent IWC surveys show an overall decrease of 30% from
the late 1980s population estimates across the entire Antarctic (Branch and Butterworth 2001, IWC 2013). This decrease may be reflecting differing year to year ice
conditions which may affect the total number of animals counted, or it may indicate
a real decline. Bioacoustics may be an alternative method for density estimation
of this highly-mobile species in these ice locked habitats (Marques et al. 2009,
Williams et al. 2014).
Most baleen whales undergo seasonal migration between breeding and feeding
grounds. The breeding season of southern baleen whales, generally, is considered to
be austral winter with conceptions occurring from July to December, with a peak
from August to October (Best 1982, Kasamatsu et al. 1995). Some groups of minke
whales may leave the Antarctic for northbound migration by February and reach
waters between 30°S and 40°S in March (Best 1982, Kasamatsu et al. 1995). Genetic
evidence suggests, however, that tropical and temperate regions are likely not a major
destination (Glover et al. 2010). Exact locations of breeding areas of the Antarctic
minke whale populations remain unknown, but they may not be as concentrated as
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those of more coastal distributed baleen whale species, such as humpback whales
(Megaptera novaeangliae) (Kasamatsu et al. 1995).
Antarctic minke whale habitat preferences are influenced by ice cover and prey
densities (Thiele et al. 2004, Murase et al. 2013). Off the WAP, Antarctic minke
whales were found in greater numbers during seasons where a sea ice boundary
existed over shelf areas (Thiele et al. 2004). Year-to-year changes in areal extent, sea
ice concentration and its formation could cause a change in spatial distribution of
Antarctic minke whales in open sea (Thiele et al. 2004, Murase et al. 2013). Antarctic minke whales share prey with other baleen whales, thus they may be partitioning
their foraging habitat to avoid direct competition (Friedlaender et al. 2006, Santora
et al. 2010). In general, visual sightings data from these regions might indicate low
detectability (Ainley et al. 2007) and create a challenge for the study of these animals
(Williams et al. 2014).
As mentioned earlier, PAM has become an important tool for monitoring cetaceans. PAM is especially useful in remote areas, during nighttime, or in adverse
weather conditions and in general for species that are difficult to study at sea (Mellinger et al. 2007a, Van Parijs et al. 2009). Long-term records of seasonal occurrence
and distribution patterns that can be obtained from PAM provide valuable insights
into the habitat utilization of vocally active species (e.g., Sirovic et al. 2004, Sirovic
and Hildebrand 2011, Gallus et al. 2012, Mussoline et al. 2012). This method has
proved to be particularly useful in the Southern Ocean, where visual surveys are
mainly conducted in open sea, and the vast majority of areas covered by sea ice are
not surveyed (Van Opzeenland 2010, Murase et al. 2013).
Identification of the source of the bio-duck call has allowed us to use this seasonally
ubiquitous sound to investigate seasonality trends in Antarctic minke whale call
production and augment our understanding of the biology for this difficult-to-study
species. In this paper, we report on the occurrence of bio-duck call and Antarctic
minke whale downsweeps from a year-long record in a coastal area in the WAP and
compare the presence of calls to sea ice concentration. In addition, we used these
long-term recordings to provide a more detailed description of the bio-duck call and
offer insights into diel call production patterns.
Methods
Passive acoustic data were recorded using an autonomous Acoustic Recording
Package (ARP), an acoustic recording device with a tethered hydrophone 10 m above
a seafloor-mounted instrument frame (Wiggins 2003). The ARP consisted of a data
logging system with a 16-bit A/D converter and 36 GB of storage capacity, a hydrophone (sensitivity 198 dB re: 1 Vrms/lPa and a 3 dB low-end roll-off at around 5
Hz), an acoustic release, two ballast weights, batteries, and flotation (Wiggins 2003).
An ARP was deployed off the WAP (65°220 S, 66°280 W) at a depth of 462 m,
sampling continuously at 500 Hz from 26 February 2002 to 18 February 2003 yielding a total of 358 d of recordings over a bandwidth of 5–250 Hz.
Collected data were converted into WAV files and processed into 5 s averaged
Long Term Spectral Averages (LTSAs) with 1 Hz frequency resolution using the
MATLAB program (Mathworks Inc.) Triton (Wiggins and Hildebrand 2007). Data
were reviewed visually by scanning LTSAs and spectrograms (fast Fourier transformation [FFT] 256 points, 80% overlap, Hanning window), as well as aurally, when a
sound of interest was identified. Sounds of interest included bio-duck calls (defined as
: SEASONALITY OF ANTARCTIC MINKE WHALE CALLS
DOMINELLO AND SIROVIC
829
series of downswept pulses) and individual downsweeps. The start and end time of
each calling encounter was logged, with individual events delineated by a period of at
least 30 min without calls.
In addition to marking the presence of calling encounters, nonoverlapping identifiable calls were further selected for detailed acoustic measurements of call characteristics. In an effort not to oversample calls from the same individual, only one call per
calling event was used for measurements. Calls were measured from a spectrogram
window of 10 s, 128-point FFT, 90% overlap and Hanning window. Frequency resolution and temporal resolution were 3.9 Hz and 0.26 s, respectively. The following
parameters were measured manually for all call types using Triton: start and end time
(s), start, end, and peak frequency (Hz) for each pulse in the sequence. Peak frequency
was defined as the frequency at the highest spectrum level over the bandwidth of the
call. Window lengths for spectral plots were 0.5 s for bio-duck variant B, 0.7–1 s for
bio-duck variants A1, A2 and C, 3 s for bio-duck variant D and 5–10 s for downsweeps. Peak frequency of calls was the only parameter obtained from the power spectrum plot in Triton, others were measured from spectrograms. From the initial
measurements, downswept pulse duration (s), number of pulses per series, bandwidth
(Hz), interseries interval (ISI) and interpulse interval (IPI) were calculated. A series
was defined as a cluster of downswept pulses separated by less than 1 s, and multiple
series made up a call. ISI was defined as the time from the start of a downswept pulse
in a series to the start of a downswept pulse in the next series. IPI was defined as the
difference between the start of one downswept pulse and the start of the next pulse
within the series. The mean and the standard deviation of each parameter were calculated. For bio-duck variants with frequency variability over the duration of the call,
rate of change in pulse peak frequency over a series and rate of change in the ending
frequency were also calculated.
Monthly encounter rates for bio-duck calls and downsweeps are reported. To investigate the relationship between sea ice cover and call occurrence, daily sea ice concentration estimates were made using Special Sensor Microwave/Imager (SSM/I) passive
microwave data obtained from the National Snow and Ice Data Center (http://nsidc
.org). Sea ice concentrations were binned to 25 km2 cells in a polar stereographic projection. We extracted daily mean values for 75 9 75 km areas centered at the ARP
using the imaging software WIM (Kahru 2015). For a more detailed description of
sea ice concentration estimates see Sirovic et al. (2004). We calculated autocorrelation
scales for each call type to ensure independence of samples. Days before the first and
after the last occurrence of Antarctic minke whale calls in the recordings were not
used in calculating autocorrelation scales. Percentage of time calling for bio-duck
calls and downsweeps were binned into 9 d and 8 d averages, respectively, for correlation with the sea ice concentration.
A chi-squared test was used to investigate diel patterns in bio-duck and downsweep occurrence. Total time spent calling (in hours) was calculated for daytime and
nighttime, where durations of day and night were based on the sunrise and sunset
times for our deployment location obtained from the U.S. Naval Observatory. If an
encounter start and end spanned across sunset or sunrise, the encounter was split
appropriately into a portion that occurred during each day period. First, daily data
were tested for homogeneity using the chi-square test of homogeneity. If the sample
was homogenous, we applied a chi-squared test on the pooled total.
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Results
The bio-duck call was the most common Antarctic minke whale call found at this
location in the WAP during 2002/2003, occurring during 22.8% of the recorded
time (1,956.3 h). These calls were detected primarily from June to October, although
a few calls were recorded in April, May, and in November (Fig. 1). Downsweeps
occurred during 2.8% of the total effort (237.3 h). They occurred intermittently
between May and February and were absent in April and March, but there was a peak
in calls in June and July (Fig. 1). Bio-duck calls recorded in the WAP could be classified into four distinct variants based on their frequency and pulse rates, with one
variant having two additional subtypes (Fig. 2, Table 1).
Bio-duck “A” type calls consisted of a series of about four pulses with peak
frequency between 130 Hz and 150 Hz. They were further classified into two
variants, A1 with a consistent number of pulses in a series and A2 with a variable
number of pulses and also of variable end frequencies. Bio-duck A1 was the most
common call found and is the call described in previous literature (Matthews et al.
2004, Risch et al. 2014a). Bio-duck A2 often had the end frequency of downswept
pulses increase and/or decrease within the same series unit, while A1 end frequency
tended to only increase within the same series unit. We consider A1 and A2 subsets
of A because on rare occasion, A2 changed into A1 or vice versa, and was seemingly
produced by the same individual based on the amplitude and timing of the call
variants. This type of gradual change was not seen in other call types.
Bio-duck B typically had more pulses per series, and an overall higher start and
end frequency compared to the other variants (Table 1). Since its start frequency was
Daily hours of encounters
a
12
9
6
3
0
Daily hours of encounters
b 24
18
12
6
0
Feb02 Mar02 Apr02 May02 Jun02 Jul02 Aug02 Sep02 Oct02 Nov02 Dec02 Jan03 Feb03
Figure 1. Average daily encounter durations (in hours) per month for minke whale
(a) downsweeps and (b) bio-duck calls. Note the difference in y-axis scale between the plots.
: SEASONALITY OF ANTARCTIC MINKE WHALE CALLS
DOMINELLO AND SIROVIC
831
Figure 2. Spectrograms of Antarctic minke whale (a) bio-duck A1, (b) bio-duck A2,
(c) bio-duck B, (d) bio-duck C, (e) bio-duck D, and (f) downsweep (128 point FFT with 90%
overlap and Hanning window) recorded off the western Antarctic Peninsula in 2002. Note (f)
downsweeps have been concatenated in time, to represent the variability of the call, and the
irregularity in the production of this call type.
very close to the recording Nyquist frequency, it is likely that the true start frequency
was not measured. While duration of the downswept pulse of variant B was similar
to those of A variants, B had a shorter IPI than A1 and a longer ISI than both A1 and
A2 (Table 1). Many B calls were detected, but most of them were faint, or overlapping with A1. Therefore, not many events were available for call characteristic measurements (Table 1). Bio-duck C was the only bio-duck variant that had a start
frequency that decreased over successive pulses in a series as well as an end frequency
and peak frequency which decreased with each pulse. Bio-duck C was not frequently
encountered. Bio-duck call D consisted of regularly repeated individual pulses, not
forming a series. Type D was the least frequently encountered call type. Because of its
infrequency, not many events were available for measurements. We consider type D
to be a bio-duck call type and not a downsweep sequence because of its regularity and
repetitive structure, which was similar to other bio-duck calls. While type D only
consists of a single pulse per series, the pulses are regular and are repeated upwards of
120 pulses per call. Of the bio-duck variants, type D downswept pulses had the longest duration (Table 1).
Minke whale downsweeps were individually occurring calls. Frequency and temporal characteristics measured during this work (Table 1) were similar to those reported
previously (Schevill and Watkins 1972). There was no evident repetitive pattern and
the number of downsweeps per event ranged from single, two, or three downsweeps
to several downsweeps (Table 1).
Bio-duck calling was homogenous (v2 = 164.02, df = 150, P = 0.79), but there
was no significant difference between nighttime and daytime calling over the entire
11
4
3
6
3
20
Call type
Bio-duck A1
Bio-duck A2
Bio-duck B
Bio-duck C
Bio-duck D
Downsweep
Start
frequency (Hz)
231 19
191 33
244a 7
122 49
155 16
126 18
41
41
13 1
41
1
—
149 19
131 22
216a 14
85 22
99 13
97 16
Peak
frequency (Hz)
116 20
101 21
177 12
64 11
52 8
55 8
End
frequency
(Hz)
0.1 0.1
0.2 0.1
0.1 0.1
0.2 0.1
0.4 0.1
0.3 0.1
Pulse
duration (s)
+5.2
—
–0.3
–6.2
—
—
Peak
frequency
change
(Hz/pulse)
+8.6
—
+0.5
–12.7
—
—
End
frequency
change
3.1 0.2
3.1 0.6
5.1 0.8
2.7 0.1
—
—
ISI (s)
For bio-duck B call, start frequency is not necessarily the call’s true start as it is nearly at the maximum of the recording bandwidth.
a
N
(events)
Number
of pulses
per series
0.4 0.1
0.3 0.1
0.3 0.0
0.4 0.1
1.7 0.1
—
IPI (s)
Table 1. Measured parameters of Antarctic minke whale bio-duck call types and downsweeps, including means ( standard deviation) of call start and
end frequency, pulse duration, peak frequency, rate of change in peak and end frequency, ISI and IPI, are shown. Rate of change in peak frequency and
ending frequency are calculated as (end value – start value)/number of pulses. Total number of samples, N, and number of pulses per series are also given.
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MARINE MAMMAL SCIENCE, VOL. 32, NO. 3, 2016
: SEASONALITY OF ANTARCTIC MINKE WHALE CALLS
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833
calling period (v2 = 1.22, df = 1, P = 0.73). The downsweep calling was not
homogenous (v2 = 51.67, df = 157, P = 0.0) and therefore chi-squared analysis was
used for each individual day, but there was no significant difference between nighttime
and daytime calling on any day. Both bio-duck calls and Antarctic minke whale downsweeps were found to be significantly positively correlated with the sea ice presence
(r = 0.56, df = 21, P = 0.005 and r = 0.52, df = 35, P = 0.001, respectively; Fig. 3).
Discussion
To our knowledge, this is the first record describing Antarctic minke whale calling
seasonality in the WAP, including the first year-long analysis of minke whale downsweep occurrence. In previous studies on Antarctic minke whales downsweeps were
a
Downsweep % of time calling
Bio−duck % of time calling
100
90
80
70
60
50
40
30
20
10
0
b 0
45
20
40
60
80
100
80
100
40
35
30
25
20
15
10
5
0
0
20
40
60
Sea ice cover
Figure 3. Sea ice concentration compared to (a) bio-duck and (b) downsweep percentage of
time calling. Note the difference in y-axis scale between the plots.
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only mentioned briefly, as a way to confirm an Antarctic minke whale is vocally
active (Schevill and Watkins 1972, Risch et al. 2014a). Fin whales (Balaenoptera
physalus), blue whales (Balaenoptera musculus), and dwarf minke whales (Balaenoptera
acutorostrata) are also known to produce downsweep calls (Gedamke et al. 2001,
Sirovic et al. 2004, Rankin et al. 2005). Downsweeps produced by fin whales average
0.7 s and sweep from 27.6 Hz to 14.9 Hz (Sirovic et al. 2004), lower than downsweeps reported here. Blue whale downsweeps are in the frequency range of Antarctic
minke whale downsweeps, but have a longer duration of about 2.7 s (Rankin et al.
2005). For these reasons we do not believe the downsweep calls we have identified to
be attributable to these baleen whale species. Humpback whales are capable of
producing downsweep like calls (Dunlop et al. 2007), and effort was made to exclude
downsweeps in presence of other humpback whale calls when conducting our
analysis. Dwarf minke whale distribution into the WAP is not known, therefore it is
possible some of the Antarctic minke whale downsweeps we have identified could be
attributed to dwarf minke whales. Antarctic minke whale downsweeps were present
throughout most months during the recording period, with few downsweeps in
January and February, none in March and April, and peaks in July (Fig. 1).
The bio-duck call was the more common Antarctic minke whale call found at this
location in the WAP, but its occurrence was more seasonal. Bio-duck calls were
detected between April and November with peak calling during July (Fig. 1). The
bio-duck call was previously suggested to be found during the austral winter and
spring in the western Antarctic (Van Opzeenland 2010) and our records extend that
occurrence into fall. Recordings of the bio-duck call in Perth Canyon off Australia
peak in July–August and again in December, suggesting the migration of this species
is complex (Matthews et al. 2004, Erbe et al. 2015). Part of the population may
undertake seasonal migrations while another part may remain in the ice, there could
be a staggered migration with seemingly continuous presence in the Antarctic
(Matthews et al. 2004, McCauley 2004, Erbe et al. 2015). However, the bio-duck
calls produced in Perth Canyon during the winter included different bio-duck
variants than the ones we encountered, suggesting the variants may represent subpopulations of the species (Matthews et al. 2004).
Even though bio-duck calls were strongly seasonal in this area, the near year-round
presence of downsweeps suggests that there were Antarctic minke whales present in
months the bio-duck call was absent. Visual surveys confirm year-round Antarctic
minke whale presence in the area (Thiele et al. 2004, Ducklow et al. 2007) with
highest occurrence close to shore and between islands (Thiele et al. 2004, Ainley
et al. 2012). Some of these inshore sightings coincide with the location of our recordings. Thus our data in combination with the year-round visual sightings suggest that
the seasonality seen in bio-duck call recordings is more likely the result of a change
in calling behavior than an indication of migration or movement out of the area. Similarly, pulse trains from minke whales (Balaenoptera acutorostrata) occur year round,
however, their seasonality can vary by location (Risch et al. 2014b).
The functionality of the bio-duck call is not known. The bio-duck call is somewhat
similar acoustically to the pulse train, another call commonly found for minke whales
in the Atlantic Ocean (Mellinger et al. 2000, Risch et al. 2013). Pulse trains are also
common in the winter and spring, and are believed to be a reproductive call, but this
has not been conclusively shown (Edds-Walton 1997). Given their similar characteristics and temporal occurrence, the bio-duck call may also be a reproductive call. It is
plausible that reproductive calling can happen in areas other than the breeding
grounds based on prior findings in other species. Humpback whales and blue whales
: SEASONALITY OF ANTARCTIC MINKE WHALE CALLS
DOMINELLO AND SIROVIC
835
are known to produce reproductive calls during migration and on feeding grounds
(Mattila et al. 1987, McSweeney et al. 1989, Clapham and Mattila 1990, Norris
et al. 1999, Clark and Clapham 2004, Oleson et al. 2007). North Atlantic minke
whales also produce pulse trains on the feeding grounds and during migration (Risch
et al. 2014b). It has been suggested that the bio-duck call may serve as a tool for navigating ice covered areas, however, this hypothesis would not explain its production
in ice-free areas (Matthews et al. 2004, Van Opzeeland 2010). A broad-ranging
breeding grounds or multiple functions of the bio-duck call would be more probable
reasons for its wide occurrence.
An additional clue to the functionality of the bio-duck call may come from the fact
that there are no significant diel patterns to this call. This may suggest this call is not
involved in feeding, since most feeding occurs during the night in this area (Konishi
et al. 2014). In the North Atlantic during the migration to feeding grounds, on the
other hand, minke whale calls occurred most commonly at night (Risch et al. 2013).
However these patterns are not consistent to all areas and diel patterns may be site
specific (Norris et al. 2012). Site specific diel patterns have been observed in North
Atlantic right whales (Eubalaena glacialis) (Mellinger et al. 2007b). Differences in diel
patterns were hypothesized to be due to prey availability, where the site with no diel
pattern observed was relatively sparse in prey availability compared to the site where
a significant diel pattern was observed (Mellinger et al. 2007b). To our knowledge
Antarctic minke whale prey studies have not been conducted during winter, but
Antarctic minke whale distribution tightly matches their prey distribution between
April and June (Friedlaender et al. 2006).
Downsweep call rates peaked in the winter during a time when sea ice forms rapidly.
If downsweeps produced by other baleen whale species function to maintain spacing
(Edds-Walton 1997, 2000) then Antarctic minke whales may utilize this call more
when there are more individuals present or when sea ice cover is more prominent. Both
bio-duck calls and downsweeps were found to be positively correlated to sea ice presence. This is in agreement with higher Antarctic minke whale visual sightings occurring in correlation with sea ice (Thiele et al. 2004, Ainley et al. 2012, Murase et al.
2013). The relationship, however, was not linear but step-like. There was a high relative frequency of time spent calling when sea ice cover was greater than 50% (Fig 3).
However, increasing sea ice concentration is not linked to an increase in calling. On
one hand, it is possible that a sea ice cover threshold needs to be reached to trigger a
vocal behavioral response. On the other hand, our sea ice data covered a large spatial
scale and may not provide a tight coupling to the location where calling minke whales
occurred. A tighter coupling of calling whales to sea ice concentration at their location
may provide a better insight to the real relationship between these two variables.
In this study, we showed year-round trends of the Antarctic minke whale bio-duck
and downsweep calls. In the future, a more detailed look at distinct patterns of bio-duck
variants, to investigate whether there are variant differences in diel patterns, seasonality,
or relationship to sea ice cover, could be illuminating for learning more on the functional importance of those variants. Additionally, investigating different recording sites
would help determine how much call variation occurs over different regions and potentially give insight into Antarctic minke whale population structure. Extending this
analysis to more years and over a large temporal scale may also explain if the observation
of few bio-duck calls produced during the austral summer (Risch et al. 2014a), not seen
in our study, could be due to a change in sea ice cover. It is possible that ecological
changes in the WAP (Ducklow et al. 2007) are also changing the calling behavior of
Antarctic minke whales. Collecting more current, long-term data at this location would
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offer insights into ways in which animals in this rapidly warming region may be
responding to environmental changes. In any case, these findings add substantially to
the paucity of data describing the acoustic behavior of Antarctic minke whales in the
WAP and will provide a useful benchmark for future comparison studies.
Acknowledgments
The data collection was supported by NSF Office of Polar Programs grant OPP 99-10007 as
part of the SO GLOBEC program. We would like to acknowledge the assistance of Raytheon
Polar Services marine and science support personnel, the Masters, Officers and crew of ASRV
L. M. Gould during LMG02-01A and LMG03-02. We also acknowledge the National Snow
and Ice Data Center for the processing and distribution of sea ice concentration data.
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Received: 26 May 2015
Accepted: 19 November 2015